Since the invention of digital holographic data storage technology it is a superior objective to increase the storage capacity within a recording medium, or to minimize the size of the recording medium for a certain quantity of information, respectively. The information is recorded in form of volumetric interference patterns inside the volume of the recording medium. In the technology of digital holographic storage, the information is arranged in the form of binary data pages, which is usually realized by a modulation of a laser beam using a spatial light modulator (SLM), i.e. an array of light modulating pixels. This laser beam is systematically scanned over the surface of the recording medium, e.g. a rotating holographic disk having a recording layer, or a holographic storage card. The increase of the storage density up to 100 bits/μm2 is due to the use of the third dimension, i.e. not only a plane surface is used for carrying the information, but a volume in the recording layer. As indicated before the data are arranged in the form of data pages containing a plurality of bits, e.g. 100 or more bits. Recovered data pages are usually analyzed using a CCD array having the same number of pixels as the SLM. Due to the high parallelism of data readout, high data transfer rates of up to 10 Gbits/s and more are achievable.
One problem encountered in holographic recording systems is cross talk from adjacent recorded holograms. The suppression of cross talk between different holograms depends on the field angle. Large cross talk leads to a low signal-to-noise ration (SNR). For example, the shift distance of shift-multiplexed holographic memory systems has to be selected according to the minimum signal to reference beam angle to avoid excessive cross-talk from the corresponding part of the data page. However, this leads to a loss in data density in other parts of the data page, as described by Steckmann et al., Appl. Opt. 40, 3387-3394 (2001). Another example for a position dependent SNR is given by Curtis et al., J. Opt. Soc. Am. A 10 2547-2550 (1993).
Orlov et al., Appl. Opt., 43, 4902-4914 (2004) describe the design and implementation of a high-data rate, high capacity digital holographic storage disk system. The data density per hologram is described as the ratio of the number of recorded data pixels divided by the storage location area. The data storage density is affected by different parameters of the system set up and the storing method. The most influencing parameters are the material of the storage medium itself, the material thickness of the storage medium, and the wavelength of the light beam, as smaller focus geometries are achievable for shorter wavelengths. Generally, for high data densities in a digital holographic storage system the amount of SLM-to-CCD pixel misdetection due to imaging distortions and aberrations has to be within very narrow boundaries, e.g. less than 0,2 pixel or better. This detection precision, along with a high numerical aperture (NA), puts stringent requirements on the quality of the imaging optics.
The imaging at large field angles, e.g. by objective systems with a high NA, is not as good as at small field angles. Consequently, the image is affected by optical aberrations, which increase with the increase of the field angle. This causes the SNR to depend on the field angle. This applies in particular for cheap optical components such as those used in current DVD players or recorders. From an economic point of view, it would be desirable to use similar plastic components also for holographic storage systems. As a result, the suppression of cross talk between different adjacent pixels on the CCD depends on the field angle. The pixel size has to be selected according to the maximum permissible cross talk, which is in general determined by the pixels with large field angle. However, this leads to a loss in data density in other parts of the data page. Therefore, the storage capacity of holographic storage systems is limited by the lowest SNR in the image plane.
US 2002/0075776, which is considered to constitute the closest prior art, discloses an apparatus for reading from and writing to holographic storage media, having a detector array with different detector pixel sizes. Pixel deformations are overcome by oversampling or by controlling the laser wavelength.